The generation of electric power (or electricity) using a Rankine cycle in conjunction with the heat produced in a solar collector system is in widespread use. The solar thermal collectors are typically of a trough, dish or central receiver (power tower) design. Of these, the parabolic, tracking trough-type collectors appear to be the most popular. Typically, these systems reflect the solar rays to a cylindrical pipe (receiver) located at the focal point of the collector. Thermal oil flowing within the pipe is heated and then circulated to a boiler, where steam is produced, typically in the temperature range of about 500° F.-700° F. There are some collector designs that are optimized to produce lower temperature thermal oil, typically at a temperature of about 400° F.
It is well known by those in the power industry that a power plant's thermal efficiency (kW/heat in) is heavily dependent on the maximum temperature of the working fluid operating in the Rankine cycle. This is commonly referred to as the Carnot principle. Thus, solar thermal power plant developers/designers have an incentive to reach the highest temperatures possible within the operating limits of the specified equipment. However, the efficiency of the collectors (heat transmitted to receiver divided by solar energy collected) is inversely proportional to the temperature of the fluid within the receiver. For example, according to data published by Sopogy, Inc., at 150° C. (about 302° F.) above the ambient temperature, the efficiency of a 900 W/m2 collector is approximately 70%, while at 300° C. (572° F.) above the ambient temperature, the efficiency falls off to about 50%. Thus there appears to be a trade-off between the power plant's thermal efficiency and solar collector efficiency such that moving to higher fluid temperatures may require a larger collector field than otherwise would have been necessary absent this additional loss.
For large-scale systems, a steam Rankine cycle is typically used. Temperatures above 500° F. are typical. The Nevada Solar One project operates at steam temperature near 700° F.
Prior plant designs have several limitations, such as diminished cost effectiveness and/or environmental stewardship. For instance, steam power plants are usually water cooled (except for very large nuclear plants) using a wet cooling tower. Cooling by air is very expensive due to the very high specific volume of steam. For example, at 100° F., water vapor has a specific volume of about 350 ft3/lb. If an air-cooled condenser were used, the tubes wherein the steam flows would have to be very large, as would all the piping. Very large pipes and tubes equate to large capital cost. The result, therefore, is that steam plant owners usually look for sites where water is available. In a solar plant, particularly those located in the desert, water is scarce. In the case of the 64 MW Nevada Solar One plant, the owners use water cooling and, as a consequence, need to provide approximately 1000 gallons per minute (“gpm”) of make-up water to replenish the amount that evaporates in the cooling tower. This is a source of major criticism to those who build solar plants to promote their environmental stewardship. In years to come, it is expected that water for plant cooling will become even more scarce and expensive.
Another example of a limitation of prior plant designs and methods is how, in a typical concentrating solar trough design, all collectors/receivers are built to supply the same high temperature fluid throughout. This results in a low temperature drop (e.g., about 150° F.-250° F.) across the receivers (temperature of fluid leaving the collector field—temperature of fluid entering to the collector field) which, in turn, increases the size of the field required compared to one where the temperature drop was greater (e.g., 300° F.). A larger temperature drop would allow a plant to operate with fewer receivers. However, as noted above, there is a trade-off between performance and economics.
Condensing steam turbines in the 50 MW class can be very large in diameter (condensing stages can be 7-8 feet in diameter), making them expensive and requiring up to three years of manufacturing lead time (Glen Davis, EVP, Ausra Inc.). The latter has the effect of adding substantial cost to the project by delaying power production for at least three years from project commencement.
Off-design conditions are another limitation of prior steam plant designs and methods. Solar energy is inherently cyclic as applied to daily generation. The transition from start-up in the morning to full plant output followed by shut down in the evening imposes a need for plants that can operate at off-design conditions. Unfortunately, steam plants do not do well in these circumstances. As the sun rises in the morning and the plant heats up, the steam turbine's inlet valve remains closed until the steam reaches full temperature and pressure. If steam were admitted to the turbine at part load conditions (lower temperature and pressure) the expansion would result in the formation of moisture (water droplets) in the condensing stages of the turbine which, in turn, would cause erosion damage to the turbine blades (buckets). Thus a standard steam plant practice is to wait for the steam to reach its design or near-design superheated conditions before opening the inlet valve to begin operation. Much of the solar energy that reaches the collectors during non-peak periods is not converted to power, resulting in loss of plant revenue. In some cases this may be mitigated by using natural gas fuel and/or thermal storage to accelerate the warm-up process, but both options may be expensive.
A fully condensing, high temperature steam plant, though having mature technology, is expensive to operate because of the need to keep the water very clean (boiler blow down, de-ionized water, condensate polishing, etc.), free of oxygen (or air), and the need to maintain a deep vacuum at the exit of the turbine. All this adds complexity, additional operator labor and increases in plant running costs.
An organic Rankine cycle (ORC) is an alternative to the steam cycle; it is so named because the working fluid is typically a hydrocarbon or hydrocarbon derivative. The ORC is much better suited to air cooling (specific volume of R245fa vapor at 100° F. is 1.22 ft3/lb; 350 ft3/lb for steam), does not operate in vacuum (the saturation pressure at 100° F. is 33.9 psia), uses smaller turbines (or expanders) requiring shorter lead times, and is less expensive to operate (closed system, hence there is no blow down, condensate polishing, DI water, etc.).
The ORC also overcomes the off-design problem by being able to operate at part load vapor conditions. This is due to the fact that organic fluids are characterized by the shape of their saturation curve, which normally results in an expansion process that reduces moisture as opposed to increasing moisture with steam. Thus, a substantial amount of power can be generated in an ORC plant during the daily warm-up and cool-down periods associated with solar energy.
However, the ORC has its limitations. For example, the ORC has a major disadvantage in not being able to achieve the high temperatures found in steam plants. ORC plants usually top out near 500° F. due to the thermal stability limits of the working fluid. For instance, at some elevated temperature, the working fluid may decompose and lose the properties of the original fluid.
Accordingly, there is a need in the art for solar thermal power plants and ORC systems and methods that overcome the limitations of prior plants, systems and methods.